This invention relates to an antenna device that is used in a receiver adapted to receive signals transmitted from an artificial satellite and, particularly, that realizes cost reduction while improving the anti-jamming performance, and further relates to a low-noise amplifier for use in such an antenna device.
As shown in FIG. 1, a related antenna device is a series-connected circuit comprising an antenna 1, an initial-stage or first-stage low-noise amplifier (hereinafter abbreviated as an “LNA”) 2, a band-pass filter (hereinafter abbreviated as a “BPF”) 3, and a last-stage or second-stage LNA 4 connected in series and feeds its output to a demodulation circuit. Since signal waves received by such an antenna device, which are transmitted, for example, from an artificial satellite or the like, are weak, the first-stage and the second-stage LNAs 2 and 4 are essential components in the antenna device.
Connected firstly to the antenna 1, the initial-stage low-noise amplifier (initial-stage LNA) 2 comprises, as shown in FIG. 2, a capacitor C and an inductor L, an impedance conversion circuit 12, and a low-noise amplification portion (hereinafter abbreviated as an “LNA portion”) 13.
For example, the LNA portion 13 comprises low-noise transistors. Each low-noise transistor may a CMOSFET (complementary metal oxide semiconductor field effect transistor) such as a GaAsFET or a HEMT (high electron mobility transistor). The capacitor C and the inductor L are required as an input matching circuit for the LNA portion 13. The capacitor C is a coupling capacitor whose capacitance is adjustable. The inductor L is used for grounding gates of the low-noise transistors used in the LNA portion 13 and may be a pattern inductor comprised of an inductor, a resistor, and so on. The input matching circuit formed by the capacitor C and the inductor L is adapted for the GPS (Global Positioning System), satellite digital radio broadcasting services, and so on in Japan and the U.S.A. and, further, for general satellite radiowave services and so on in Europe, Russia, and so on. That is, as shown in FIG. 3, it is designed so that the initial-stage LNA 2 directly connected to the antenna 1 is adapted to receive signals from satellites over a wide frequency range on the order of gigahertz (GHz).
The impedance conversion circuit 12 is necessary for achieving impedance matching with an output of the antenna 1. The LNA portion 13 has a high input impedance and, therefore, the impedance matching with a 50-ohm impedance of the antenna output is required on the input side of the low-noise transistor of the LNA portion. For this purpose, the impedance conversion circuit 12 has a pattern having a quarter-wavelength (λ/4) that is connected to the input side of the LNA portion 13.
In the foregoing antenna device, the initial-stage LNA 2 connected to the antenna 1 has a pass band which is set to cover all frequency bands intended for general use. Therefore, the pass band of the initial-stage LNA 2 includes such a frequency band that corresponds to a specific use other than that of a receiver incorporating the antenna device. For example, 800 to 900 MHz for cellular phones with a high frequency of use may be included in the pass band of the initial-stage LNA 2 so as to be jamming waves. Consequently, it is expected that many jamming waves are mixedly fed into the LNA portion 13 used in the initial-stage LNA 2. Therefore, in order to prevent this, it is necessary that a trap circuit or a band-elimination filter (BEF), adapted for the specific use to block the jamming waves, be provided on the input side of the initial-stage LNA 2.
An antenna device adapted to avoid such jamming waves due to the general-use bands is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2005-109602 (JP 2005-109602 A2). In this antenna device disclosed in JP 2005-109602 A2, as shown in FIG. 4, first and second trap circuits 5L and 5H adapted to reject the passage of specific band frequencies of 800 to 900 MHz and 1.8 to 1.9 GHz, respectively, which may be jamming waves, are added at an input portion of an initial-stage LNA 2 connected to an antenna 1. Accordingly, as shown in FIG. 5, an input signal in 1.5 GHz band intended for use is supplied to the initial-stage LNA 2 without containing jamming waves at nearby frequencies and passes through the initial-stage LNA 2 so as to be produced.
Further, as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2004-260555 (JP 2004-260555 A2), there is a receiver in which a filter is interposed between an antenna and a low-noise amplifier (LNA) when the input power of interference waves, whose influence is large due to wideband operation, is large.
As described above, in the related antenna devices, the circuit element for rejecting the jamming waves is required per specific use on the outside of the low-noise amplifier. Consequently, an increase in size and cost of the antenna devices cannot be avoided.
Therefore, it is an object of this invention to provide an antenna device that has a jamming wave rejecting function and further that realizes cost reduction.